ConspectusHighly dispersed transition-metal Lewis acid centers (e.g., Zn, Co, Y, La, Fe, Sn, Hf, and Zr) and Lewis acid-anchored noble metal centers (e.g., Pt–Zn, Pt–Sn, Pt–Fe, Rh–Zn, and Rh–Co) supported on siliceous zeolites are promising catalysts for a number of industrially important reactions, such as alcohol dehydrogenation, aldol condensation, alkane dehydrogenation, and olefin hydroformylation. In this Account, we describe the preparation and characterization of Lewis acid centers grafted onto hydrogen (H)-bonded silanol groups present in zeolites as well as Lewis acid-anchored noble metal centers and discuss the mechanism and kinetics for different reactions occurring over each type of center. We show that isolated and nested Lewis acid centers can be created by the reaction of hydrated cationic species with H-bonded silanol groups on dealuminated beta (DeAlBEA) or Silicalite-1 zeolite. We then demonstrate that isolated and nested Lewis acid centers are effective catalysts for light alkane dehydrogenation. Nested Lewis acid centers can also serve as efficient anchoring sites for dispersing noble metals such as Pt and Rh to generate bimetallic centers that exhibit superior catalytic performance relative to monometallic Pt and Rh for reactions such as alkane dehydrogenation and olefin hydroformylation. Finally, we summarize our recent investigations of isolated and nested Lewis acid centers and the Pt- and Rh-based bimetallic centers as catalysts for ethanol conversion to 1,3-butadiene (ETB), acetone conversion to isobutene (ATI), propane dehydrogenation to propene (PDH), n-butane dehydrogenation to butene and 1,3-butadiene (BDH), and ethene hydroformylation to propanal. We show that the activity of Lewis acid centers for these reactions is affected by their local coordination environments. In particular, we highlight the significance of H-bonding between hydroxyl groups connected to Lewis acid centers in an open configuration (M–OH) and silanol groups on zeolite supports to generate (≡SiO)xMn+–OH···(O(H)–Si≡)y structures, which exhibit aldol condensation activities that are higher than that of (≡SiO)xMn+–OH sites. These studies demonstrate that siliceous zeolites rich in H-bonded silanol groups can be utilized to create highly dispersed Lewis acid centers and can be further employed as an anchoring platform for noble metal atoms to construct atomically dispersed bimetallic centers. Both the chemical structure and the local coordination environment of these centers significantly influence their catalytic performance.

The electrochemical reduction of carbon dioxide using renewably generated electricity offers a potential means for producing fuels and chemicals in a sustainable manner. To date, copper has been found to be the most effective catalyst for electrochemically reducing carbon dioxide to products such as methane, ethene, and ethanol. Unfortunately, the current efficiency of the process is limited by competition with the relatively facile hydrogen evolution reaction. Since multi-carbon products are more valuable precursors to chemicals and fuels than methane, there is considerable interest in modifying copper to enhance the multi-carbon product selectivity. Here, we report our investigations of electrochemical carbon dioxide reduction over CuAg bimetallic electrodes and surface alloys, which we find to be more selective for the formation of multi-carbon products than pure copper. This selectivity enhancement is a result of the selective suppression of hydrogen evolution, which occurs due to compressive strain induced by the formation of a CuAg surface alloy. Furthermore, we report that these bimetallic electrocatalysts exhibit an unusually high selectivity for the formation of multi-carbon carbonyl-containing products, which we hypothesize to be the consequence of a reduced coverage of adsorbed hydrogen and the reduced oxophilicity of the compressively strained copper. Thus, we show that promoting copper surface with small amounts of Ag is a promising means for improving the multi-carbon oxygenated product selectivity of copper during electrochemical CO₂ reduction.

Generation of solar fuels through photo-catalytic reduction of CO 2 could simultaneously address both the need for sustainable and affordable energy sources and the necessity of reducing global CO 2 footprint. Discovering and designing new catalysts that have both high activity and selectivity for CO 2 reduction is the critical barrier to overcome in order for solar fuel generation processes to commercialize. So far, copper is the sole material known to efficiently catalyze conversion of CO 2 to considerable amounts of C2 hydrocarbon products over time. However, CO 2 reduction on Copper exhibits a high overpotential plus the added disadvantage of producing a mixture of several products including hydrogen. 1,2 Thus, it is of vital importance to understand the so far ambiguous and poorly understood catalytic CO 2 reduction mechanism on copper. In this work, a combination of various in-situ X-ray spectroscopy techniques are utilized to probe the electrode/electrolyte interface and provide insight into the reduction reaction.X-ray absorption spectroscopy (XAS) provides an element-specific probe of the conduction band via a core-level excitation into unoccupied electronic states and can reveal both oxidation state and details of the electronic structure. Operando Grazing incidence XAS at beamline 11-2 of SSRL is utilized to study the electrode surface at the metal K- and L-edges. A specially designed 3D printed flow cell maintainsa 300 micron liquid layer above the catalyst surface, enabling XAS characterization of the catalyst surface during electrochemistry in grazing incidence mode. The grazing incidence geometry provides a probe depth of about 5 nm into the catalyst, hence providing a highly surface sensitive spectroscopic tool. Preliminary measurements were conducted on the relatively inert surface of AuPd alloy catalyst. This is the first step in determining the feasibility of the study before examining the more complicated copper system. Distinct reversible shifts are observed in spectroscopic features with applied potential, which are believed to be due to H + intercalation into Pd phase. (Figure 1) Further, operando soft X-ray spectroscopy measurements at C-edge are conducted at beamline 8.0.1 of ALS to detect intermediate carbon species. Electron yield soft XAS data are collected at a depth of 1 nm above the electrode surface in a specially designed in-situ electrochemical flow cell 3 . Strong 1s to π* transition in C edge spectra provides fingerprint for different products. First principles DFT calculations are conducted to characterize features obtained through these XAS measurements. (Figure 2) Combination of these spectroscopic techniques with electrochemical and theoretical calculations will paint a comprehensive picture of the exact CO2RR mechanism on Cu surface, which then can be used to design next generation photo-catalysts. References: Y. Hori, K. Kikuchi and S. Suzuki, Chem. Lett., 1985, 1695–1698. K. P. Kuhl, E. R. Cave, D. N. Abram and T. F. Jaramillo, Energy Environ. Sci., 2012, 5, 7050-7059. J.J. Valesco-Velez, C.H. Wu, T.A. Pascal, L.F. Wan, J. Guo, D. Pendergast and M. Salmeron, Science, 2014, 346, 831-834. Figure 1

A reactor/membrane system was designed, built, and tested for improved furfural production from xylose.

An interest in the on-purpose production of 1,3-butadiene (1,3-BD) has grown, as a consequence of the decline in naphtha cracking for the production of ethene and propene, products that can now be produced economically by thermal dehydrogenation of ethane and propane contained in natural gas. In this study, the mechanism and kinetics of n-butane dehydrogenation to 1,3-BD are explored over atomically distributed Pt sites grafted onto dealuminated zeolite BEA (DeAlBEA) in the form of (Si-O-Zn)4-6Pt complexes. Reaction of n-butane dehydrogenation carried out at 823 K with 2.53 kPa n-butane/He and a weight-hourly space velocity (WHSV) of 14.5 h-1 produced 1,3-BD with a turnover frequency of 0.45 mol 1,3-BD (mol Pt)-1 s-1. Space-time studies and identification of the reaction intermediates suggest that n-butane first undergoes dehydrogenation primarily to 1-butene, which then rapidly isomerizes to produce an equilibrated mixture of 1-butene and 2-butene. 1-Butene then undergoes secondary dehydrogenation to produce 1,3-BD. We report, here, a detailed study of the kinetics of n-butane dehydrogenation to butenes and 1-butene dehydrogenation to 1,3-BD over isolated Pt sites. Both reactions exhibit a Langmuir-Hinshelwood dependence on n-butane and 1-butene partial pressures, respectively. Comparison of effective forward rate constants of n-butane dehydrogenation to butenes (k1f) and butene dehydrogenation to 1,3-BD (k2f) shows that the isolated Pt sites grafted onto DeAlBEA exhibit a very high activity for sequential dehydrogenation of n-butane to 1,3-BD relative to other Pt-based catalysts previously reported.

An artificial-photosynthesis device is a multicomponent system composed of various components including perhaps light absorbers, electrocatalysts, membranes or separators, and electrolytes in a specific system geometry. The overall solar-to-fuel conversion efficiency of such a system depends on the performance and materials properties of the individual components as well as the design of the system. In this talk, we will cover recent modeling of various motifs of such artificial-synthesis systems. In particular, we will examine modeling and experimental results for particle-based devices for solar-hydrogen production as well as vapor-feed devices for solar water splitting and electrochemical carbon-dioxide reduction. Mathematical modeling is ideally suited to examine the various tradeoffs and determine design targets and feasibility. Z-scheme particle-suspension reactor designs consisting of freely suspended semiconductor particles in an electrolyte to drive solar water splitting could be cost-effective alternatives to produce renewable hydrogen. In this work, we develop a device-scale model to evaluate the effects of coupled light absorption, electrolyte species transport, and reaction kinetics on overall reactor performance and ability to sustain rector operation via diffusion. We also extend this work by numerically investigating particle-scale and -size effects on colloidal stability, light absorption and scattering, and charge-carrier transport across the semiconductor/cocatalyst/electrolyte interface. Within the Joint Center for Artificial Photosynthesis (JCAP), we utilize continuum-scale modeling of the various components in order to determine design tradeoffs of vapor-feed or gas-diffusion electrode systems. Such systems can provide routes towards optimizing local reaction conditions and overcoming inherent liquid-phase transport limitations for carbon-dioxide reduction as well as solar water splitting to produce hydrogen. For the latter, integrated architectures can be used that are more stable than those in liquid environments. Overall, the functioning of both vapor feed and particle systems will be explored.

Because of their relatively low cost, ethane and propane derived from shale gas are the currently preferred feedstocks for the production of aromatics. Ga-exchanged H-MFI zeolite (Ga/H-MFI) exhibits high activity and selectivity for light alkane dehydroaromatization, a process that involves alkane dehydrogenation to form alkenes, followed by alkene oligomerization and cyclization, and further dehydrogenation of the resulting products. Recent work has shown (Phadke et al. Characterization of Isolated Ga3+ Cations in Ga/H-MFI Prepared by Vapor-Phase Exchange of H-MFI Zeolite with GaCl3. ACS Catal. 2018, 8, 6106–6126; Phadke et al. Mechanism and Kinetics of Propane Dehydrogenation and Cracking over Ga/H-MFI Prepared via Vapor-Phase Exchange of H-MFI with GaCl3. J. Am. Chem. Soc. 2019, 141, 1614–1627; Phadke et al. Mechanism and Kinetics of Light Alkane Dehydrogenation and Cracking over Isolated Ga Species in Ga/H-MFI. ACS Catal. 2021, 11, 2062–2075; Mansoor et al. ACS Catal. 2018, 8, 2146–6162) that Ga3+ ([GaH]2+ and [Ga(H)2]+) sites catalyze the initial dehydrogenation of alkanes; however, the role of Ga3+ sites in the subsequent alkene oligomerization step requires clarification. In this work, the kinetics of ethene oligomerization over Ga/H-MFI were investigated as a function of Ga loading, feed space time, temperature, and ethene partial pressure. The presence of Ga3+ sites gives rise to enhanced rates of ethene oligomerization and higher selectivities to butene and hexene relative to H-MFI. However, selective titration of Brønsted acid sites with NH3 reveals that, in the absence of Brønsted acid sites, [GaH]2+ and [Ga(H)2]+ cations do not contribute appreciably to the higher activity of Ga/H-MFI. Similarly, in situ Fourier-transform infrared spectroscopy shows that the reaction pathway for ethene oligomerization over Ga/H-MFI involves the same intermediates as that over H-MFI. The higher ethene oligomerization activity and selectivity to even-carbon-numbered alkenes of Ga/H-MFI stems from cooperative effects between Ga3+ sites and Brønsted acid protons.

We assess the accuracy of popular nonempirical GGAs (PBE, PBEsol, RPBE) and meta-GGAs (TPSS, revTPSS, and SCAN) for describing chemisorption reactions at metal surfaces. Except for RPBE, all the functionals tend to overbind the adsorbate significantly. We then propose a nonempirical meta-GGA, denoted as RTPSS, that is based on RPBE in the same way that TPSS is based on PBE. The RTPSS functional remedies the overbinding problem and improves the description of chemisorption energies. As an example of an application of RTPSS, we study the adsorption of CO on Cu surfaces (a notably difficult problem for semilocal functionals) and find that RTPSS is the only tested functional that predicts accurate chemisorption energies and the preferred adsorption site of CO. Although RTPSS gives an accurate description of chemisorption, nonlocal correlation may be necessary to describe physisorption if long-range van der Waals interactions are involved (however, this is true for semilocal functionals in general). We suggest that RTPSS can be a useful meta-GGA for studying chemisorption processes and mechanisms of heterogeneous catalysis.

Significance Chemical storage of solar energy can be achieved by electrochemical reduction of CO 2 to CO and H 2 , and subsequent conversion of this mixture to fuels. Identifying optimal conditions for electrochemical cell operation requires knowledge of the CO 2 reduction mechanism and the influence of all factors controlling cell performance. We report a multiscale model for predicting the current densities for H 2 and CO formation from first principles. Our approach brings together a quantum-chemical analysis of the reaction pathway, a microkinetic model of the reaction dynamics, and a continuum model for mass transport of all species through the electrolyte. This model is essential for identifying a physically correct representation of product current densities dependence on the cell voltage and CO 2 partial pressure.

Abstract Growing concern with the effects of CO 2 emissions due to the combustion of petroleum‐based transportation fuels has motivated the search for means to increase engine efficiency. The discovery of ethers with low viscosity presents an important opportunity to improve engine efficiency and fuel economy. We show here a strategy for the catalytic synthesis of such ethers by reductive etherification/O‐alkylation of alcohols using building blocks that can be sourced from biomass. We find that long‐chain branched ethers have several properties that make them superior lubricants to the mineral oil and synthetic base oils used today. These ethers provide a class of potentially renewable alternatives to conventional lubricants produced from petroleum and may contribute to the reduction of greenhouse gases associated with vehicle emissions.

Growing concern with the environmental impact of CO₂ emissions produced by combustion of fuels derived from fossil-based carbon resources has stimulated the search for renewable sources of carbon. Much of this focus has been on the development of methods for producing transportation fuels, the major source of CO₂ emissions today, and to a lesser extent on the production of lubricants and chemicals. First-generation biofuels such as bioethanol, produced by the fermentation of sugar cane- or corn-based sugars, and biodiesel, produced by the transesterification reaction of triglycerides with alcohols to form a mixture of long-chain fatty esters, can be blended with traditional fuels in limited amounts and also arise in food versus fuel debates. Producing molecules that can be drop-in solutions for fossil-derived products used in the transportation sector allows for efficient use of the existing infrastructure and is therefore particularly interesting. In this context, the most viable source of renewable carbon is abundantly available lignocellulosic biomass, a complex mixture of lignin, hemicellulose, and cellulose. Conversion of the carbohydrate portion of biomass (hemicellulose and cellulose) to fuels requires considerable chemical restructuring of the component sugars in order to achieve the energy density and combustion properties required for transportation fuels-gasoline, diesel, and jet. A different set of constraints must be met for the conversion of biomass-sourced sugars to lubricants and chemicals. This Account describes strategies developed by us to utilize aldehydes, ketones, alcohols, furfurals, and carboxylic acids derived from C₅ and C₆ sugars, acetone-butanol-ethanol (ABE) fermentation mixtures, and various biomass-derived carboxylic acids and fatty acids to produce fuels, lubricants, and chemicals. Oxygen removal from these synthons is achieved by dehydration, decarboxylation, hydrogenolysis, and hydrodeoxygenation, whereas reactions such as aldol condensation, etherification, alkylation, and ketonization are used to build up the number of carbon atoms in the final product. We show that our strategies lead to high-octane components that can be blended into gasoline, C₉-C₂₂ compounds that possess energy densities and properties required for diesel and jet fuels, and lubricants that are equivalent or superior to current synthetic lubricants. Replacing a fraction of the crude-oil-derived products with such renewable sources can mitigate the negative impact of the transportation sector on overall anthropogenic greenhouse gas (GHG) emissions and climate change potential. While ethanol is a well-known fuel additive, there is significant interest in using ethanol as a platform molecule to manufacture a variety of valuable chemicals. We show that bioethanol can be converted with high selectivity to butanol or 1,3-butadiene, providing interesting alternatives to the current production from petroleum. Finally, we report that several of the strategies developed have the potential to reduce GHG emissions by 55-80% relative to those for petroleum-based processes.

Abstract Cation exchanged-zeolites are functional materials with a wide range of applications from catalysis to sorbents. They present a challenge for computational studies using density functional theory due to the numerous possible active sites. From Al configuration, to placement of extra framework cation(s), to potentially different oxidation states of the cation, accounting for all these possibilities is not trivial. To make the number of calculations more tractable, most studies focus on a few active sites. We attempt to go beyond these limitations by implementing a workflow for a high throughput screening, designed to systematize the problem and exhaustively search for feasible active sites. We use Pd-exchanged CHA and BEA to illustrate the approach. After conducting thousands of explicit DFT calculations, we identify the sites most favorable for the Pd cation and discuss the results in detail. The high throughput screening identifies many energetically favorable sites that are non-trivial. Lastly, we employ these results to examine NO adsorption in Pd-exchanged CHA, which is a promising passive NO x adsorbent (PNA) during the cold start of automobiles. The results shed light on critical active sites for NO x capture that were not previously studied.

In order to understand the remarkable activity of α-Bi<sub>2</sub>Mo<sub>3</sub>O<sub>12</sub> for selective oxidation and ammoxidation of propene, the propene activation ability of four molybdenum-based mixed metal oxides - Bi<sub>2</sub>Mo<sub>3</sub>O<sub>12</sub>, PbMoO<sub>4</sub>, Bi<sub>2</sub>Pb<sub>5</sub>Mo<sub>8</sub>O<sub>32</sub>, and MoO<sub>3</sub> - was investigated using density functional theory. Propene activation is considered to occur via abstraction of a hydrogen atom from the methyl group of physisorbed propene by lattice oxygen. For each material, the apparent activation energy was estimated by summing the heat of adsorption of propene, the C-H bond dissociation energy, and the hydrogen attachment energy (HAE) for hydrogen addition to lattice oxygen; this sum provides a lower bound for the apparent activation energy. It was found that two structural features of oxide surfaces are essential to achieve low activation barriers: under-coordinated surface cation sites enable strong propene adsorption, and suitable 5- or 6-coordinate geometries at molybdenum result in favorable HAEs. The impact of molybdenum coordination on HAE was elucidated by carrying out a molecular orbital analysis using a cluster model of the molybdate unit. This effort revealed that, in 5- and 6-coordinate molybdates, oxygen donor atoms trans to molybdenyl oxo atoms destabilize the molybdate prior to H addition but stabilize the molybdate after H addition, thereby providing an HAE ~15 kcal/mol more favorable than that on 4-coordinate molybdate oxo atoms. Bi<sup>3+</sup> cations in Bi<sub>2</sub>Mo<sub>3</sub>O<sub>12</sub> thus promote catalytic activity by providing both strong adsorption sites for propene and forcing molybdate into 5-coordinate geometries that lead to particularly favorable values of the HAE. (Graph Presented).